As voltage regulation criteria for digital circuits such as CPUs become more stringent, the demand for high dynamic performance power converters increases. Among the many characteristics of dynamic performance, output voltage overshoot/undershoot and recovery time are often considered the most important. In general, the output voltage deviates under load current change, or input voltage change. To improve the dynamic response of a DC-DC converter, the switching frequency and/or output filter can be altered.
For example, increasing the switching frequency may improve the dynamic response of a converter having a small output capacitance. However, increasing the switching frequency complicates the design of the converter, and as the switching frequency increases, the efficiency of the converter decreases eventually to an unacceptable level.
Increasing the output capacitance of a converter can help to maintain the output voltage during a sudden load current change. However, this strategy requires a very large output capacitor (e.g., 5,000 to 10,000 μF), which is bulky and expensive, and consequently is not practical. Alternatively, reducing the output inductance of a DC-DC converter can improve its dynamic response. However, such a reduction results in an increase in output voltage ripple. The increased voltage ripple will in turn reduce the room for the output voltage drop during dynamic response. In addition, a larger ripple current through the filter inductor will result in a larger RMS current through the power switches of the converter, which will reduce the overall efficiency of the converter under steady state operation.
It is evident that such options for improving the dynamic response of a DC-DC converter do not provide a viable solution.
Various control methods have been proposed for improving the dynamic response of a power converter. Use of current mode control may provide a faster dynamic response than conventional voltage mode control in situations where only a small change in load current occurs. On the other hand, voltage mode control has superior dynamic response when a large transient occurs. More importantly, use of current mode control in high current applications may be impractical because of the limitations on accurate and efficient current sensing at high current.
For example, energy balancing techniques [1]-[2] and second-order switching surfaces [3] were proposed to minimize the settling time and the voltage overshoot/undershoot due to a load transient. Other schemes include a switch for shorting the output inductor of a Buck converter was disclosed in U.S. Pat. No. 6,271,651, issued Aug. 7, 2001 to Stratakos et al. This method provides a relatively simple way to increase the output current during a step increase in load current. A method of improving transient response of a Buck converter, but only during a negative load current step, was proposed in U.S. Pat. No. 6,753,723, issued Jun. 22, 2004 to Zhang. U.S. Pat. No. 7,002,817, issued Feb. 21, 2006 to Lipcsei, disclosed a further method based on comparing the output voltage of the converter with a reference voltage. Others have proposed digital control for power converters (e.g., U.S. Pat. No. 7,019,505, issued Mar. 28, 2006, and U.S. Pat. No. 7,038,438, issued May 2, 2006, both to Dwarakanath et al.).
None of the schemes mentioned above is capable of providing the transient response required for high performance power converters. In particular, none of these schemes properly address the voltage overshoot caused by a step-down load current transient, which may be more than five times as large as the corresponding voltage undershoot caused by a positive current step of equal magnitude. To address the large overshoots typical of voltage regulator module (VRM) applications, auxiliary circuits have been proposed for the Buck converter.
For example, in [4]-[5], a transformer was connected across the impedance of the output trace of a Buck converter to inject/absorb excess load current to improve the dynamic performance. In [6], an auxiliary switch was used to bypass the output inductor of a Buck converter to provide a very low inductance path to the output. The switch remains full-on for the duration that the output voltage deviation exceeds a pre-determined threshold. An auxiliary switch in series with a small inductor was used in [7] to recover excess current to the input during step-down load transients. The circuit also provided a low-impedance auxiliary path for step-up load transients. The auxiliary circuit was controlled using a differentiator in an attempt to instantaneously track the capacitor current. In [8], the output of an isolated DC-DC converter was connected through an auxiliary circuit (similar to [7]) to a voltage rail (fed by the rectified voltage of the secondary winding) to inject/absorb excess current. The auxiliary circuit was controlled linearly based on the magnitude of the output voltage. An auxiliary circuit (similar to [7]) was connected to the output of a Buck converter in [9]. The switch is turned full-on for the duration that the output voltage deviation exceeds a predetermined threshold.
While such topology modifications may improve the dynamic response of a DC-DC converter during a load transient, they suffer from at least one of the following: complicated transformer design due to high-frequency operation; auxiliary switch control susceptible to noise caused by auxiliary switching; unpredictable auxiliary switching frequencies; no direct current-mode control of the auxiliary circuit resulting in unpredictable and potentially damaging currents; and high auxiliary peak current to average current ratio resulting in necessity of relatively large auxiliary switches for desired dynamic performance.